The function of heat exchangers is to move as much heat as rapidly as possible from a first fluid to a second fluid of different temperature. This involves maximizing the temperature difference at the boundary between the cooled and cooling fluids, maximizing the heat exchange surface area and minimizing the effect of the boundary between the cooled and cooling fluids. Some conventional heat exchangers maximize the temperature difference by using pipes to juxtapose the cooled and cooling fluids in cross-flow or counterflow arrangements and maximize the surface area by increasing the number and decreasing the spacing between pipes and by projecting fins into the fluids. To minimize interference with heat transfer across the cooled-cooling fluid boundary, relatively expensive metals of high thermal conductivity are often used.
A difficulty with using straight pipe to conduct cooled or cooling fluids, however, is that the layer of fluid adjacent the surface of the pipe is stagnant and resists heat flow more than a moving fluid because the stagnant layer only slowly mixes with the moving inner layer.
One method to circumvent this problem is to place fins on the pipes to increase the heat exchanger surface area. However, since a temperature gradient always exists along the fin itself, its effectiveness in transferring heat is limited in proportion to its length and cross section; furthermore, it should be fused to the pipe boundary to be effective. These restrictions greatly increase the cost of construction and maintenance of good heat exchangers.
Many conventional heat exchangers reduce these costs by increasing the number and decreasing the spacing between heat exchange pipes, thereby increasing the overall size of the heat exchange unit.
The current design choice for heat exchangers is, therefore, between a small expensive heat exchanger which may be difficult to maintain, or a larger less efficient heat exchanger. This choice is impractical for many heat exchanger needs.
A commercial need exists, and has long existed, for a compact and inexpensive heat exchanger as described below.